Modern fiber-optic communications systems use a method called wavelength division multiplexing or WDM to send massive amounts of information at extremely high data rates over a single optical fiber. In these WDM systems there are many optical wavelengths (also called optical channels) that are used to carry the information. The optical power at each of these wavelengths co-propagates with the power at the other wavelengths on a single optical fiber cable. At certain points along the optical fiber, it may be necessary to remove and/or add an optical channel. This can happen, for example, in a long-distance communication system whenever the fiber cable enters a city. It can also happen within a city (or metropolitan area network) when optical channels are routed by using their wavelength. Devices that perform this function are called add/drop filters.
The general principle of these devices is illustrated in FIGS. 1 and 2. FIG. 1 illustrates the properties of the drop function. A wavelength division multiplexed signal 100 is introduced to the #1 port 102 of the add/drop filter 104. A designated wavelength, here xcex3, is intended to be dropped. The dropped wavelength xcex3 will be output through port #3106. The remainder of the spectrum, that is xcex1, xcex2, and xcex4, will be output through the #2 output port 110.
The add function of the add/drop filter is illustrated in FIG. 2. The partial spectrum, xcex1, xcex2, xcex4, is input as input wave 200 (to #1 port). The wavelength to be added, xcex3, is input through #4 port 202. The complete spectrum with all of xcex1-xcex4 is output through #2 output port 210.
Several different kinds of add/drop filter devices have been proposed. Of these approaches one that is most nearly related to this invention is described in xe2x80x9cUltracompact Sixe2x80x94SiO2 Micro-Ring Resonator Optical Channel Dropping Filtersxe2x80x9d, by Little et al (herein xe2x80x9cLittlexe2x80x9d). In that approach, two waveguides are prepared on a wafer using lithography and etching techniques. These waveguides are situated on opposite sides of a disk that has also been defined using lithography and etching. The disk is designed to sustain optical modes, characterized by their resonant wavelength and their quality factors or xe2x80x9cQxe2x80x9d. The positions of the waveguides permit coupling of optical power between the waveguides and the disk. When the wavelength of this optical power coincides with a resonant wavelength of the disk, optical power can be transferred between the waveguides. This permits realization of the add/drop function.
The Little reference describes a monolithic add-drop device where key components of the device are fabricated onto a single semiconductor chip. Devices like these have several limitations. First, because the Little device is fabricated as waveguides and other parts on a chip, the waveguides and the disk resonator are etched or otherwise defined into the chip. Although fabrication of this kind of monolithic-optical-element lends itself well to mass production, it has drawbacks. There can be a large insertion loss associated with coupling any waveguide created on a wafer to optical fiber. Several undesirable decibels of loss are typical for the fiber-to-chip coupling. Also, the manufacturing process that couples optical fibers to on-chip waveguides is costly. Hence, the cost associated with producing fiber-coupled devices such as in the Little reference can be high. Another disadvantage of the Little device is parasitic optical loss induced during the fabrication process, because of unwanted optical scattering from imperfections at lithographic-defined interfaces. Such loss can adversely affect propagation through the device as well as the quality factor or Q of the resonator.
The optical Q is a figure of merit often cited in optical resonators and provides a reference point as to the quality of a resonator. The optical Q or quality factor of a resonator mode is defined as Q=xcexd/xcex94xcexd where xcexd is the optical frequency of the given mode while xcex94xcexd is the modes linewidth. The Q""s of the resonators of the present invention can exceed 1 million. High Q is not only important in establishing a basis for comparison of resonator quality, but also affects the way in which the add/drop device functions. In general, higher Q resonators can provide more flexibility in design, and can allow for a wider range of system applicationsxe2x80x94even beyond the application cited above to add/drop filters.
The present system teaches a special kind of resonator-based all-fiber optic bi-directional coupler in which optical power is resonantly transferred from a first optical fiber to a second or vice versa by way of coupling to a high-Q optical cavity. One application is to wavelength-division-multiplexed optical communications systems where a version of the device can function as an add/droop filter. Another application would use the ultra-high Q properties of the filter for high-resolution optical spectrum analysis.
This application defines an optical device that has an optical fiber, which has a first thinned portion, formed such that a fraction of the guided optical power propagates outside of the first thinned portion; and a resonator, coupled to said first thinned portion, such that optical power can be transferred to the resonator. The resonator can be spherical, or disk shaped, for example.
A second optical fiber, having a second thinned portion, can be also coupled to said resonator, such that power can be transferred between the first fiber and the second fiber.